1. Field
The disclosure relates to the field of tire manufacture, particularly that of the manufacture of treads for tires.
More particularly, the disclosure is concerned with the manufacture of the flexible walls placed in a groove of a tire tread pattern.
2. Description of Related Art
The grooves in the tread pattern are there to drain and remove water when the tire is running over wet or soaking ground. These grooves cause some of the running noise phenomena insofar as they constitute zones in which air builds up under pressure generating sound waves. The grooves are arranged in the circumferential direction and in the axial direction, and their layout, dimension and number have been studied in detail by tire tread pattern designers.
In order to reduce the level of tire running noise, while at the same time maintaining the water clearance qualities, it is known practice in the art to position thin flexible walls in a direction transverse to the direction of the grooves. These walls, also known as flexible membranes or flaps, extend transversely from one of the walls of the groove and have the specific feature of maintaining their initial shape closing the groove when the tire is running on dry ground in order to block the spread of sound waves, and of being able to flex and thus open the groove when the tire is running on wet ground, in order to allow water entering the drainage groove to be removed.
Publications FR 2 715 891, GB 2 450 723 or even EP 908 330 describe various embodiments of these flexible walls.
It has been found that the most common forms of embodiment are flexible flaps or walls extending from the bottom wall of the groove, because of the absence of undercuts that oppose the demolding of the tire after the vulcanizing step. The wall of the flap has one or more radial slits allowing the said flap to pivot about its point of attachment to the wall of the groove as the flap passes from an open position to a closed position. The thickness of such flexible walls or flaps varies between 0.2 mm and 2 mm for a tire fitted to passenger cars.
In practice, the molds intended for molding the tires comprising such flexible flaps comprise closed cavities having the shape and dimensions of the flaps which cavities are made within the molding ribs intended to mold the drainage grooves.
However, creating these cavities with the required degree of precision comes with a great many difficulties, particularly when use is made of mold production techniques involving pouring molten material over an expendable mold, generally made of plaster, and in which the slenderness of the elements makes the latter excessively fragile.
A solution proposed in document FR-2946915-A1, consists in creating inserts comprising cutting reservations positioned at right angles to the direction of the groove and arranged within the molding ribs of the liner elements. Although these work satisfactorily, it has nevertheless been found that it was difficult to obtain, by molding, small thickness flexible flaps separated from the lateral wall of the groove by slits even finer than the thickness of these flaps, this being mainly because of problems of getting the rubber to flow into very narrow gaps. Moreover, demolding flexible flaps with inclined walls proves difficult and carries the risk of tearing the flaps. What is more, as the slits are separated from the lateral wall of the groove by a distance generally of between 0.1 and 0.2 mm, the tolerances imposed on the manufacture and fitting of the said inserts prove to be highly restrictive.